The objective of the proposed research is to study by micropuncture techniques the factors which are responsible for modulation of renal tubular transport of phosphate in dogs with normal and reduced renal function. Emphasis is placed on the investigation of the mechanism involved in functional adaptation of phosphate transport in chronic renal disease. Unilateral remnant kidney models will be used for micropuncture studies and will be induced by ligating 3/4 to 5/6 branches of the main renal artery in one kidney. Changes in phosphate transport in the proximal and distal tubule of the remnant kidney will be studied before and after removal of the contralateral kidney. In addition, various experimental maneuvers will be carried out in these models to investigate the role of parathyroid hormone and altered sodium transport in the pathogenesis of phosphate adaptation in chronic renal failure. These include acute and chronic parathyroidectomy, extracellular volume expansion, clamping of the contralateral renal artery and parathyroid hormone administration. It is hoped that the proposed projects will lead to further clarification of the underlying pathophysiologic mechanisms involved in the altered phosphate transport of chronic renal disease and thus may contribute to improving the methods of treatment in such patients.